Field of the Invention
The present invention relates to control of a motor for driving a hydraulic pump in a hydraulic unit for a machine tool.
Description of the Related Art
In a hydraulic unit for supplying oil pressure by rotating a motor connected to a hydraulic pump, it is often a case that the motor is rotated at a constant high speed and the oil pressure is adjusted to a desirable level, by means of an oil pressure relief valve or the like. In this case, as oil from the relief valve is intentionally leaked, high power consumption results with the motor. Further, as the motor is rotated at a high constant speed, a loud noise is emitted. To address the above, various attempts have been made in controlling a motor for driving a hydraulic pump.
FIG. 7 is a block diagram showing a conventional control device of a motor 9 for driving a hydraulic pump. An oil pressure sensor 12 is mounted on a hydraulic circuit to which a hydraulic pump 10 and a load 11 such as a hydraulic cylinder or the like are connected. Using a hydraulic determination value Pd from the oil pressure sensor 12 as a feedback value, a subtractor 2 obtains a hydraulic deviation relative to a hydraulic instruction value Pc outputted from a superior control device 1. Based on the hydraulic deviation, a speed instruction operation unit 3 outputs a speed instruction value Vc through proportional-integral control. Meanwhile, a motor position determination unit 8 is mounted on the motor 9, which is connected to rotate the hydraulic pump, and determines a position determination value. The position determination value is differentiated by a differentiator 13 to output a speed determination value Vd of the motor 9. A subtractor 5 obtains a deviation between the speed instruction value Vc and the speed determination value Vd of the motor 9, and outputs the result as a speed deviation. Based on the speed deviation, a torque instruction operation unit 6 outputs a torque instruction Tc through proportional-integral control. Based on the torque instruction Tc, a current control unit 7 including an inverter flows a current into the motor 9 to control the motor 9.
As the motor 9 is controlled based on the oil pressure determination value Pd determined by the oil pressure sensor 12, it is possible to keep the rotation of the motor 9 to a minimum required rotation and resultantly to reduce power consumption as well as noise.
However, according to the conventional art shown in FIG. 7, in the case where a sharp change in load should occur, response may be delayed. As a result, it takes time to attain a desired oil pressure, which resultantly causes a problem of a longer than expected response time in relation to a load.